1. Field of the Invention
The present invention is related to a DC brush motor and a permanent magnet used within. In particular, the present invention is related to a DC brush motor and associated permanent magnet whose small size and high torque are made possible by the use of an anisotropic rare earth bonded magnet as the permanent magnet. The present invention is very effective for example in 1-300 W high-performance small-size DC brush motor applications.
2. Background Art
Prior to 1960, small-sized motors did not use magnets, but were induction brush motors. From 1960, with the appearance of affordable ferrite magnets with a maximum energy product (BHmax) on the order of 4 MGOe, small-sized brushless motors with a power consumption on the order of 1-300 W appeared, and have been used for the last 40 years. The configuration of these motors comprises 2-pole or 4-pole sintered ferrite magnets tiled on the inside perimeter of the housing, in the center of which is an electromagnetic rotor core wound with coils. When the motor is driven, the direction of the current flowing through the coils is changed via the brushes arranged on the rotational axis, and the Lorentz forces, which arise from the interaction between the current and the magnet field derived from the peripheral sintered ferrite magnets, induce rotation of the electromagnetic rotor core.
In recent years, there has been a demand for the miniaturization of such small-sized motors, however this has not been realized because sintered ferrite magnets with thin enough wall thickness cannot be manufactured due to the shrinkage of sintered ferrite magnets during sintering. Moreover, high-output motors could not be realized as sintered ferrite magnets have a low attractive force.
In addition, if one attempts to make a large-size motor in order to achieve high output, there is no alternative but to make a 4-pole motor, as the arc length is too great for a 2-pole motor. In this case of a 4-pole motor using sintered ferrite magnets, the size and weight are increased, and it is not possible to improve the motor performance index (torque constant/volume). Furthermore, as the shape of sintered ferrite magnets differs depending on the environmental conditions, such as humidity and the sintering conditions, it is difficult to achieve tiled sintered ferrite magnets of exactly the same dimensions. In addition, it is necessary to individually arrange said sintered ferrite magnets in said motor housing. Because of this, the problem of squeaking and rattling can occur due to uneven torque resulting from errors in symmetry of the magnetic field made during precision arrangement. In the latter part of the 1990's, an anisotropic bonded magnet with superior molding properties, and superior magnetic properties of a maximum energy product (BHmax) no less than 14 MGOe, or four times that of a ferrite magnet, appeared on the scene and investigation into its application to motors began.
However, these magnets were not adopted because when motor manufacturers simply tried to replace the ferrite magnets of conventional small-sized brush motors with these magnets having four times the maximum energy product, the motor properties only increased on the order of 20%, and because the back yoke needed to be doubled, the size actually increased. In addition, as the motor properties depend on several factors such as armature shape and properties, back yoke thickness and material, coils, etc., the increase in properties could only be expected to be on the order of 20% and therefore these magnets have not been adopted in recent years.